A feedwater pump can be used to supply feedwater into a steam boiler. To protect the feedwater pump from damaging conditions during operation, a flow through the feedwater pump is maintained above a recommended minimum value. The recommended minimum value can be met by having a minimum flow recirculation valve recirculate water in the pump as needed. The minimum flow recirculation valve is conventionally controlled based on measurements of the feedwater pump delivery pressure or flow.
A conventionally used flow measurement technique in industrial applications is calculation of the fluid flow rate based on readings of the pressure loss across a pipe restriction using a differential-pressure flowmeter. Differential-pressure flowmeters can rely upon a pressure connection on both sides of a flow element, such as an orifice plate or venturi flow nozzle. The connections are called impulse lines, and typically contain root isolation valves at the piping connection to the main feedwater piping. If these impulse lines become partially or completely blocked by corrosion products or an incorrectly positioned root valve, the flow indication may be incorrect. If the flow indication reads higher than the actual flow, the recirculation valve control loop may act based on the incorrect flow reading, causing the recirculation valve to close to a position that allows less flow through the pump than the recommended minimum, and pump damage can occur.